This invention relates to a process for producing copper-based alloys having high strength and high electric conductivity that are suitable for use as materials in fabricating electric and electronic parts as typified by leadframes.
With the recent expansion of the electronics industry, the use of materials for electric and electronic parts such as leadframes has increased and they are required to have higher reliability in characteristics in addition to the need for lower costs.
The term "leadframe" as used herein means "a monolithic frame structure that supports IC leads during and after the fabrication of ICs". The leadframe is required to have the following characteristics:
(1) High Thermal and Electric Conductivity
One of the primary functions of the leadframe is to prevent the deterioration of Si chips by dissipating the heat generated in the chips. For efficient heat dissipation, the leadframe must have good thermal conductivity. In addition, it must have good electric conductivity in order to reduce the generation of heat in leads.
A liner relationship generally holds between thermal conductivity and electric conductivity, so the performance of the leadframe in this aspect can conveniently be evaluated by measuring its electric conductivity.
(2) High Strength
The leadframe must have sufficient strength to support IC leads during and after the fabrication of ICs. Criteria for evaluation of the leadframe in this aspect include high tensile strength, high proof stress and adequate stiffness.
(3) Adequate Heat Resistance
The leadframe may be subjected to some heating during or after the fabrication of ICs. Hence, the lead frame must have sufficient heat resistance to insure that its strength will not deteriorate under such thermal loads. In practice, however, in order to make leadframes having very high heat resistance, the temperature for annealing the frame material must be increased and other phenomena that are economically disadvantageous would occur. Hence, a leadframe that does not soften upon heating at 350.degree. C. for several minutes will suffice for practical purposes.
(4) Good Workability for Bending
Most leadframes are worked for bending at the leads and hence good workability for bending is another requirement that must be satisfied by the leadframe. This property may be evaluated by a V/W bend test or an alternating bend test.
(5) Good Platability and Solderability
The leadframe is often plated with Ag/Au on the inner lead and solder on the outer lead. Therefore, good platability and solderability as well as the high weather resistance of the plate or solder are necessary for the leadframe.
The leadframe is required to possess the characteristics described above.
Copper-based alloys such as brass, phosphor bronze, beryllium bronze, low tin copper and Fe-P containing copper are commonly used as materials for making electric and electronic parts that possess those necessary characteristics but all of them are short of completely satisfying the performance requirements for the leadframe.
Co-P containing copper has been proposed in Unexamined Published Japanese Patent Application Nos. 34632/1980, 44553/1980, etc. as an inexpensive copper-based alloy having superior characteristics. However, the so far proposed Co-P containing copper alloys are inappropriate in terms of either the Co and P contents or the thermo-mechanical treatment applied and, therefore, they have not possessed sufficiently good characteristics to justify their use as materials for electric and electronic parts such as leadframes.